1. Field of the Invention
The present invention relates to an optical pickup having two light sources whose wavelengths differ from each other, and more particularly, to an optical pickup having two light sources and an objective lens.
2. Description of the Related Art
An optical pickup includes a laser diode, an objective lens and a photodetector, which is used for recording and reproducing information such as video, audio or data at high density. An optical disk used as a recording medium comprises a plastic or glass medium having a uniform thickness through which light is incident from a laser diode, and a signal recording surface which is located on the medium and on which information is recorded.
Recently commercialized digital video disks called digital versatile disks (DVDs) have a reduced substrate thickness of 0.6 mm but a high recording density, compared with an existing compact disk (CD) or a CD-recordable (CD-R) having a substrate thickness of 1.2 mm.
Optical pickups adopting various methods using the light beam whose wavelength is 650 nm have been proposed in order to make such DVDs compatible with the existing CD. There are an annular shielding method, a two-lens method, a hologram method and a liquid crystal shutter method as the proposed methods.
A CD-R has a reflection coefficient of 70% or more with respect to the light beam having a wavelength of 780 nm. The reflection coefficient is the same as that of the existing CD. However, a CD-R has a reflection coefficient which is lowered down to 10% or less with respect to the wavelength of 650 nm used for DVDs. As a result, the optical pickups using the above-described methods can be compatible with both a DVD and a CD, while it is impossible to make the above-described optical pickups be compatible with both a DVD and a CD-R.
To solve this problem, there have been proposed various techniques for constituting an optical pickup which can use both 780 nm wavelength light and 650 nm wavelength light. One of the proposed techniques uses two optical pickups which includes an objective lens and a photodetector. The other thereof uses an optical pickup including two light sources, an objective lens and a photodetector.
Since the first technique uses two optical pickups, it can be compatible with a DVD, a CD and a CD-R, but it increases manufacturing cost and makes it difficult to be miniaturized. The second technique integrates an objective lens and two light sources in an optical pickup. In such a combination, a number of optical components are needed. Consequently, it is difficult not only to reduce the number of components, but also to increase miniaturization.